The present invention relates to a technology for transmuting transuranium elements and more particularly to a transuranium transmuting reactor core for transmuting the transuranium elements at a fast reactor and also to a transuranium elements transmuting fuel pin and fuel assembly charged into a reactor core of a fast reactor.
A spent fuel discharged from a thermal reactor such as boiling water reactor or the like includes transuranium elements (hereinafter called TRU elements) such as neptinium-237 (.sup.237 Np), americium-241 (.sup.241 Am), americium-243 (.sup.243 Am), curium-242 (.sup.242 Cm), curium-244 (.sup.244 Cm) and others which are high-level radioactive wastes, and in minor actinides (hereinafter called MA elements) present after eliminating plutonium (Pu) from the TRU elements, there exists elements such as .sup.237 Np, .sup.241 Am, .sup.243 Am or the like having an extremely long half life such as 2.14 million Years, 432 years, 7,380 years, which cannot be quenched within a short period of time. Thus, it is desired that the MA elements are transformed into elements with a short half life through a nuclear transmutation in a short period of time.
A prior art includes technique for transmuting the TRU element which comprises using a fast reactor extremely high in a neutron energy as compared with a thermal reactor and subjecting the TRU elements charged into a fuel charged in a core of the fast reactor to a nuclear transmutation ((1) "Conceptional Design Study on Actinide burning Fast Reactor", T. Osugi et al., JAER1-M 83-217, issued by Japan Atomic Energy Research Institute in December 1983; (2) "Transmutation of Transuranics in FBR", A. Sasahara, T. Matsumura, F7, Fall Meeting Reports, Atomic Energy Society of Japan, 1988).
The prior art TRU elements transmuting comprises transmuting the aforementioned MA elements by causing a transmutation shown in FIGS. 9A to 9C to the typical MA elements of .sup.237 Np, .sup.241 Am and .sup.243 Am which are main objects of transmuting at a fast reactor core.
In FIGS. 9A to 9C, F.P. denotes fission products, and elements given in a square border around indicates that of being easy to cause a fission against a neutron energy in the fast reactor, namely, that its energy averaged fission cross-sections are about 1 burn or over.
The prior art TRU elements transmuting process utilizes a feature of the fast reactor core effectively, and the feature comes in:
(1) Since a neutron energy of the fast reactor core is high, a neutron capture is hard to occur in .sup.237 Np, .sup.241 Am and .sup.243 Am and the like, and thus an evil influence of the fast reactor on a neutron economy according to the charging of the TRU elements into the reactor core is relatively small (a neutron capture cross-section getting small according as the neutron energy becomes high as shown in FIG. 18).
(2) The fast reactor is generally high by about 1 digit in a neutron flux level as compared with the thermal reactor, therefore the TRU elements can be subjected to a nuclear transformation even if a fission and neutron capture cross section on an energy average is small, and thus a high transmuting efficiency of the TRU elements is ensured.
In the prior art transmuting of the TRU elements, nothing has been taken particularly into consideration for charging amount of the TRU elements charged into a fast reactor core and its distribution in core when carrying out a transmuting of the TRU elements. Still, however, only a self-evident technical care on charging the core with the TRU elements as much as possible has been considered for enhancing a transmuting efficiency of the TRU elements.
However, if the fast reactor core is charged with the TRU elements as much as possible, then the following problems are capable of resulting therefrom.
(1) If the MA elements to be transmuted is added to uranium-plutonium mixed fuel, a melting point of the mixed fuel lowers. Then the melting point drop is capable of causing a fuel melting, thus a measure such as lowering a reactor power or the like will be necessary for avoiding the fuel melting, which may deteriorate the transmuting efficiency of the MA elements.
(2) As will be apparent from FIGS. 9A to 9D, the typical MA elements to be transmuted is generally hard to bring about a fission, and hence is transformed into fissionable elements by a neutron capture. Accordingly, if the fast reactor core is charged with the TRU element excessively much, then, as shown in FIG. 19, an amount of fissionable elements produced newly by the neutron capture of the MA element according to a neutron irradiation comes to exceed fissionable elements transmuted by fission, thus an excess reactivity of the fast reactor increasing.
Consequently, if the charging amount of the TRU elements and its distribution are not specified properly, an excessive change or distortion may arise on a reactor power distribution and a neutron flux distribution, thus leading to problems on safety and characteristics of the reactor.
(3) The TRU elements to be transmuted are easy to cause an alpha-decay in most cases, and an alpha ray energy emitted at the time of the alpha-decay is relatively high at 4 to 6 MeV generally. Accordingly, if the MA elements are added much to a fuel, a calorific value and a source intensity of gamma ray, neutron and others become excessive from the state of a fresh fuel before loading into the fast reactor core. Further, at the time of assembling, storage and transportation of new fuel assemblies in which the MA elements are enclosed, a heat removing of the alpha ray energy becomes difficult and the fuel overheats to lead to a failure in a worst case.
(4) When charging a fast reactor uniformly with the TRU elements to be transmuted at the core with a core for which a plutonium enrichment is one kind as a base, a radial distribution of the power density, namely a radial power distribution during operation of the reactor becomes small according as it comes outside, as shown in FIG. 20, therefore a transmuting efficiency of the TRU element and a plant power generation efficiency being unsatisfying.
(5) When charging the reactor uniformly with the TRU elements at the core with the fast reactor core for which a plutonium enrichment is two or more than two kinds as a base, a radial power distribution of the core is improved as compared with FIG. 20 by an adjustment of the plutonium enrichment, a flatting requirement can thus be satisfied, however, as shown in FIG. 21, for example, there arises a portion where the power distribution largely fluctuates according to burn-up.
On the other hand, a flow rate of a coolant flowed for cooling down the fast reactor core is constant through the lifetime of a reactor plant. The flow rate of the coolant to fuel assemblies is set adaptively to the time when the power is maximized. Thus, when the output distribution fluctuates largely according to the burn-up of the fuel, a heat removing efficiency deteriorates, a heating efficiency gets lowered furthermore, which is not preferable from the viewpoint of an economical operation of the reactor plant.